The objectives of the proposed research are to identify genes controlling replication during meiosis and to determine the role of these genes in the biochemical reactions of replication. Genes controlling replication (in Saccharomyces cerevisiae) will be detected by the isolation of temperature sensitive mutants with lesions blocking pre-meiotic DNA synthesis. The mutants are obtained by screening mutagenized clones for thermosensitive defects in induced meiotic recombination. Based on prior results a significant fraction of recombination deficient isolates, when tested by isotope incorporation, should have specific lesions in pre-meiotic replication. After genetic analysis, to determine dominance and segregation, suitable mutants are organized into a minimum number of functional groups (i.e., genes) primarily by genetic complementation tests. These experiments provide the basic genetic background for subsequent research. Our second objective, a start in identificatton of the biochemical reaction(s) controlled by each gene, begins with a determination of the thermosensitive period, representing the temporal span of a specific gene function. Genes with thermosensitive periods coincident with replication are further analyzed for the effects of the restrictive temperature on the kinetics of DNA synthesis. This provides a preliminary division of the genes into those which affect precurson (deoxynucleotide) formation or polymerization, from those affecting replication initiation.